[Comparative myeloarchitecture of the planum temporale in the human brain]

The studies of the hemispheric functional asymmetries in human brain began last century with Dax, Gall, Brocca and Wernicke's observations, demonstrating that language functions belong specially to the left hemisphere. Recent anatomic review were published by Flechsig and von Economo; Geschwing and Levitsky (1968) gave to these observations statistical meaning by showing that the cortex situated behind the transverse gyrus (planum temporale) is larger in the left hemisphere in 65 % of the brains, larger to the right in 11 % and equal in 24 % of the cases studied, These results were confirmed by many authors. If we consider that in 4 % patients, aphasia is due to damage in the right hemmisphere and we homologize this to the 10 % of brains with planum temporale larger to the right we see that 20 % of the cases having equal planum (left and right) possess left hemisphere dominance. To resolve this question we studied the planum temporale cortex in eight adult brains. Four of them had a larger planum to the left, and the other four were equal left and right. This work was perfomed using Weigert's method for staining myelin fibers. It was observed in all the cases that the left planum (compared with the right) had much more myelin fibers in the second and third cortical layers and more radial fibers too. It can be concluded that: 1) the left brain hemisphere is more recent phylogenetically than the right one, because it has much more myelin in its supragranulous layers; 2) this greater supragranulous complexity is related to the ability to elaborate the language performances. 3) myelin stain methods are better than macroscopic observations to determine hemispheric dominance.